1. Field of the Invention
The present invention relates to a cellulose acylate solution for film production, and especially to a cellulose acylate solution for film production that is used in a polarizing filter and a liquid crystal display.
2. Description Related to the Prior Art
Cellulose acylate film is used for photographic film, animation film and so on, and is widely used in optical fields because of properties, such as transparency, adequate moisture permeability and high mechanical strength, and dimensional resistance, which do not depend on the humidity and the temperature so much. Recently, as the demand for the liquid crystal display becomes larger, the performance thereof has advanced and the liquid crystal display becomes larger, it is remarkable that the demands of the protective film for a polarizing filters used in the liquid crystal display remarkably increases. Recently, it is required to show image more finely for the liquid crystal display. Therefore, it becomes more necessary to increase the smoothness and to reduce the foreign materials in the cellulose acylate film to be used in the liquid crystal display.
Cellulose acylate film is often produced by a solution casting method, in which a film having extremely high smoothness is produced. However, the high smoothness causes the adhesion of different surface portions of the film in winding the produced film around a winding spindle. When the film is used, it is unwound to peel the adhered surface portions thereof, and thereby an unnecessary large force is applied to the film surface. Thus the film has peppered surface with deformation or unevenness and is deformed, which causes an extreme decrease the function of the protective film for the polarizing filter.
In order to prevent the adhesion, the film surface needs to have adequate roughness, and therefore particles called matting agents are contained in the film. When the content of the matting agent is adequate, the damage of the film, such as the deformation and the like, is prevented.
In the matting agents there are several sorts of metal oxides, metal salts, organic materials, polymers and the like, and they always contain clusters (coarce particles) formed by aggregation. When particles and such clusters are mixed as foreign materials in the film, the foreign materials cause a damage of the film. Further, when the filtration is made to remove the clusters from a polymer solution (hereinafter dope), the clusters occlude the filter pores immediately, and therefore the serial production can not be made stably. Accordingly, the matting agent used for the dope is rarely used without the dispersing process to micro particles by any method.
There are micro particles, each of which has a lot of hydroxyl groups on a surface thereof, and therefore has hydrophilic property. Such micro particles easily aggregate in the dope in which a hydrophobic solvent is used, and a representative example thereof is SiO2 micro particles. In order to reduce the aggregation, the Japanese Patent Laid-Open Publication No. 7-11055 (Pages 2–4, FIG. 2) supposes that the surface of each particle may have methyl groups to provide the hydrophobic property for the micro particles. In this case, the methylation of the surface is made. Further, Japanese Patent Laid-Open Publication No. 2001-2799 supposes that the surface of each particle may have alkyl groups and the number of carbons in each alkyl group on the surfaces may be at least 2 and at most 20 so as to provide the hydrophobic property for the micro particles. In this case, the alkylation of the surface is made.
By the way, in the solution casting method, the dope of the cellulose acylate solution is prepared from cellulose acylate and several sorts of additives. The dope is cast onto a substrate with use of a casting die. When the dope on the substrate to form a polymer film, the polymer film is peeled from the substrate and dried in a drying process. As the substrate, a continuously rotatable drum or a continuously movable belt is used.
The dope prepared through dissolution of cellulose acylate and the additives to the solvent usually contains much undissolved foreign materials, such as impurities and gel-like materials. When the film is produced by casting such dope, the undissolved foreign materials remain also in the film to damage the surface conditions of the film. Accordingly, the quality of the film as the product becomes lower, and the film cannot be used. Therefore the filtration is performed at least once before the casting.
In the filtration, several sorts of filtrers are used depending on characteristics of the dope and the required properties. As materials of the filters, there are filter cloth, filter paper, metal mesh, metal fiber, non-woven cloth and the like. Further, a criterion for the small content of the undissolved foreign materials is extremely strict in case of the dope of cellulose acylate solution used for the film production. Therefore the plural filtration devices are connected in series, such that the filtration of the dope may be made plural times to improve the property of the dope.
The filter used for the filtration has the filter pores being very narrow passage of the dope. When the dope flows through the filter pores in performing the filtration, the materials having larger diameter than the filter pores are removed from the dope. Further, in some filters, the diameters of the filter pores are not uniform, and the passages of the filter pores are complexly bent and branches. Accordingly, also the particles having the smaller diameter than the filter pores are caught or trapped at branching or bending parts. Thus the foreign materials are removed by the filtration to obtain a clean filtrated solution.
However, in the hydrophobilization of the surfaces of SiO2 particles, the continuous existence or generation of the aggregation is hardly prevented, and the produced film is not adequate to use in the liquid crystal display showing images highly fine.
Further, the micro particles are transported through the filter along the flow line of the dope. On the upstream side of the filter, the flow line thickens at the filter, and therefore the distance between the micro particles becomes smaller to progress the aggregation more. This phenomenon cannot be prevented by the hydrophobilization as above described, since the surfaces of the micro particles are not entirely hydrophobilized or the inside of the particles appears in dispersing the particles to form an interface to the solvent.
As described above, parts of the clusters formed through this aggregation are trapped by the filter material, and the ratio of the occluded filter becomes higher and the filtration pressure becomes larger in the continuous filtration. When the filtration pressure becomes higher, some of the trapped particles are pressed out from the filter and are mixed in the filtrated solution flowing in the downstream side from the filter. Thus the particles pressed out from the filter are contained in the produced film, and the quality of the product becomes lower.